In wireless systems, multiple antenna technologies may be used to improve performance. MIMO (multiple-input multiple-output) is a multiple antenna technology that typically provides increased performance when compared to a single antenna implementation. MIMO may be implemented as either open loop or closed loop.
In open loop MIMO, the communications channel does not utilize explicit information regarding the propagation channel. Open loop MIMO techniques include, for example, Space Time Block Coding (STBC), Spatial Multiplexing (SM-MIMO), and Collaborative Uplink MIMO. The WiMAX standard includes two versions of Open Loop MIMO techniques referred to as Matrix A, which includes the STBC technique, and Matrix B, which includes the SM-MIMO technique.
In the case of closed loop MIMO, the transmitter collects information regarding the channel to optimize communications to the intended receiver. Closed loop MIMO implementations typically use techniques, such as Maximum Ratio Transmission (MRT) or Statistical Eigen Beamforming (EBF) techniques, both of which are simply referred to as “beamforming.”